Thixotropic oxidizer propellant mixtures



United States Patent Int. Cl. C06b 1/00 US. Cl. 149-17 8 Claims This invention relates to liquid oxidizer propellant mixtures, and more particularly to liquid oxidizer propellant mixtures having the property of being unpourable under storage or low shear conditions and of becoming fluid and flowing under increased shear.

Liquid propellant systems have a number of advantages in rocket propulsion, for example, such as higher specific impulse than solid propellants; ease of throttling, shutdown, and restarting; and simplicity of vector control. They have, however, certain disadvantages which are associated with their low viscosity, such as potential toxicity and fire hazard created by spilling and the problem of sloshing during flight; instability of slurry systems containing highly energetic finely divided particulates which settle out of the liquid; evaporation difiiculties during storage, etc. These shortcomings may be counteracted through a type of gelation of the liquid propellant which provides solid-like immobility during shipboard or other storage as well as the ability to shearthin to apparent viscosities approximating those of the unmodified liquids within the operating range of the rocket engine injector. This type of gelation is accomplished by the addition to the liquid propellant of substances which do not react with the liquid so as to either prevent imparting of the requisite property of low-shear immobility and high-shear thinning or diminish appreciably the energy content of the liquid.

This invention has an object the provision of novel liquid oxidizer propellant mixtures.

This invention has as another object the provision of liquid oxidizer propellant mixtures having good storage, pumping, and metering characteristics.

Other objects will appear hereinafter.

The liquid oxidizers of interest in rocket propulsion, for example, because of their relatively high specific impulse and/or reactivity with fuels, are normally liquid at room temperature and are required to withstand storage in the temperature regime from 65 F. to 165 F. (-53.8 to 73.89 C.). They comprise, for example, liquids such-as red fuming nitric acid, inhibited red fuming nitric acid, various interhalogen compounds (such as bromine trifluoride, bromine pentafiuoride, iodine trifluoride, iodine pentafiuoride, etc.), and mixtures of interhalogen compounds. Chlorine trifiuoride is sometimes dissolved in the interhalogen oxidizers to create mixtures having more desirable physical properties.

These liquids present considerable hazard in the event of seeping leakage from the storage tank in the rocket or gross leakage incident to rupture of the tank, such leakage of these materials occasioning vaporization of the liquid, the large surfaces thereby generated tending to produce toxic or explosive conditions. For example, bromine trifluoride in the presence of water vapor will produce a violent reaction and form copious quantities of hydrogen fluoride and hydrogen bromide. A further problem in handling these materials is their tendency toward hypergolic ignition if they are mixed prematurely with combustible or fuel materials; also, detonations have been reported when sufiicient quantities have mixed. Propellant mixtures provided by the present invention, therefore, are of considerable importance because they retain suitable end use factors such as are involved in flow induction and injector flow while, at the same time, furnishing resistance to leakage, to premature ignition, etc.

It has been discovered that carbon black of a particular kind, described hereinbelow, is a chemically compatible additive suitable for imparting to liquid oxidizer propellants the desired adequate yield stress for storage immobility and low apparent viscosity at high gear, notwithstanding the fact that the mixture of oxidizer and carbon black is theoretically unstable system and would not ordinarily be expected to form a stable system such as a system having a gel or gel-type consistency. Use of the below-designated carbon black to form the propellant mixtures of the present invention provides, not only mixtures having the requisite storage and flow property, but also propellant mixtures having higher specific impulse than are proxided by liquid propellant oxidizers unlike those of the present invention.

It is essential for the purposes of the present invention that such carbon black 'be made from acetylene gas by continuous thermal decomposition and comprise colloidal sized particles of carbon joined together in a chainlike fashion (for description of the carbon 'black utilized in the present invention, see brochure 1 entitled Shawinigan Black published by Shawinigan Chemicals Limited, P.O. Box 6072, Montreal, Canada). Thus, it has been demonstrated by electron micrographs at a magnification of fifty thousand times that the particles of the carbon black utilized in the mixtures of the present invention are joined together in a chainlike or fibrous fashion so that the mass of the black is a persistent three-dimensional structure, unlike those of most other carbon blacks.

The carbon black used in the compositions of the present invention is commercially available in two bulk densities, namely as fifty percent compressed black having a bulk density of 6.25 pounds per cubic foot and as one hundred percent compressed black having a bulk density of 12.5 pounds per cubic foot. For the properties of the carbon black utilized in the present invention, see the following table:

Compressed A.S. Value (cc./5 grams) 1 30 22 Electrical resistivity (ohm-in (at apparent density,

44 lb./cu. ft. 0.030 0.032 Apparent density (lb./cu 6.25 12. 5 True density (g./ml.) 2

Particle size:

Mean value (A.U.) 4205525 Standard deviation (A.U.) 250 Range (A.U.) 50-2000 Surface area (sq. m./g.):

Electron microscope Nitrogen adsorption 64. 5 Color index (Cabot nigrometer) 93 pH value 7-8 Percent carbon. 99. 5+ Percent moisture 0. 04 Percent volatile 0. 06 Percent ether ext 0. 03 Percent ash 0. 04

See also United States Patent 2,121,463 (S. A. Wisdom to Shawinigan Chemicals Limite(l) The carbon black used in connection with the present invention is not readily wetted with water; however, traces of acid, acetone, sugar or wetting agent overcome this resistance to wetting. The carbon black is completely insoluble in the liquid oxidizer propellants and does not chemically react 'with them.

Since the extent of unpourability under zero or low flow (i.e., the yield stress value) of the liquid oxidizer propellant mixtures of the present invention increases with concentration of the additive, and since the compositions of the oxidizer propellants vary and thereby aifect the concentration of additive necessary to obtain a given yield value for a given oxidizer, and since the extent of unpourability and yield value may be varied to suit different requirements (as, for example, to withstand certain acceleration forces, vibrations, etc., under end use conditions), the concentration of the carbon black may be varied over a broad range. However, in most situations, the carbon black should be present in admixture with the oxidizer propellant within the range of about 2 to weight percent and preferably between about three and seven weight percent; for this concentration range, the yield stresses obtained lie between about 40 and 1500 dynes per square centimeter, that is, from a consistency less stiff than a gelatin dessert to that of stiff grape jelly. Once the yield value has been exceeded, the transition from extremely high viscosity at low shear rates to low viscosity at high shear rates (as, for example, a shear rate range of from 1() to 7 x10 reciprocal seconds (seerwith shear stresses of 900 to 16,000 dynes per square centimeter) is rapid, shear thinning to apparent viscosities of 0.07 to 0.22 poise having been observed.

It has also been found that, in the course of the preparation of the liquid oxidizer mixtures of the present invention, it is possible to add other substances in the form of coarsely or finely powdered solids for various reasons, such as for the purpose of enhancing desirable activity of the oxidizers, and that the liquid oxidizer mixtures of the present invention permit the suspension of these powdered solids, the relative weight percent of the carbon black being adjusted if necessary to provide the requisite degree of suspension and shear-thinability. For example, suitably protected (i.e., coated, for example) solid particulate light metals or light metal hydrides may be added to form new monopropellants.

It is to be noted that the carbon black used to form the liquid oxidizer mixtures of the present invention is not added in sufiicient quantity to appreciably adversely affect the normal properties of the liquid oxidizer propellant and that it is not destroyed or dissolved by the liquid oxidizers. The amount of carbon black added should generally be at the lowest possible level which confers the desired characteristic upon the oxidizer, it being understood that flow initiation can be adjusted for a given situation by the amount of carbon black which is added. However, if carbon black is desired in the mixture for added specific impulse, then, of course, the highest possible amount of carbon black which confers the desired characteristic upon the oxidizer is to be added.

In addition, the propellant mixtures of the present invention may include additives which aid in the blending between the oxidizer and the carbon black, such as surface-active agents and the like, although this is not generally necessary since the suitable carbon black material normally disperses readily in the oxidizers. These agents must be compatible with the liquid oxidizers, i.e., not destroyed or altered by the oxidizer. When used, such agents are generally present in the propellant mixtures of the present invention in relatively low weight percentages, which percentages are always less than the amount of carbon black and are generally less than one weight percent of the mixture. Ordinary surface-active agents, such as the alkyl sulfonates, fatty acid alcohol amine compounds, alkyl aryl polyether alcohols, ethylene oxide-fatty alcohol condensates, etc., are not suitable for use with oxidizers containing fluorine (including inhibited red fuming nitric acid, which is red fuming nitric acid to which about one-half of one percent of hydrofluoric acid or ammonium hexafluorophosphate has been added for minimizing tank corrosion). In such cases, a surface-active material having oxidation resistance may be used, such as the fiuorochemical FC-95, an anionic surfactant product of the Minnesota Mining and Manufacturing Company, St. Paul, Minn.

The blending between the oxidizer and the carbon black should be effected with the oxidizer in the liquid state, and with the blending being achieved under vigorous agitation to achieve an intimate mixture for best results. That is, substantially complete dispersion of the liquid oxidizer propellant and the carbon black (and the surface-active material or additional material requiring suspension in the mixture if these are used) must be effected. This can be accomplished in many cases by shear working in conventional equipment. However, ultrasonic dispersion may be employed advantageously, especially in the case of mixtures comprising other particulate solids in addition to the carbon black and liquid oxidizer propellant, in order to shorten preparation time, reduce carbon black concentration, etc.

The order of addition of the components is generally not critical so long as an intimate mixture is obtained. However, it should be noted that it is sometimes efiicacious to employ a technique involving mixing of the carbon black with a little of the liquid oxidizer before adding the remainder of the liquid oxidizer in order to form more readily the mixtures of the present invention.

The propellant mixtures of the present invention have good storage characteristics, and when stored remain in gel or gel-like form for extended periods of time. In connection with the storage of this material, it is to be noted that evaporation and reaction with moist air can occur only at the periphery of the mixture in its noor low-flow condition, so that, if leakage should occur, any tendency toward filling magazines with toxic or explosive fumes is minimized.

The propellant mixtures of the present invention have satisfactory physical and chemical stability as determined by several methods, including behavior during storage at various temperatures. Thus, the propellant mixtures of the present invention were observed during extended storage at a low temperature of 51 C. (60 R), (the temperature of a Dry-Ice box without regulation), and at ambient temperatures of 2030 C. (6886 F.), and at a high temperature of 50 C. (122 F.), and during storage in a pressure vessel at C. (167 F.).

Rheological investigation of the liquid oxidizer propellant mixtures of the present invention in two types of viscometer (a conicylindrical viscometer operating in a low shear-rate range so that yield value and initial flow behavior could be determined, and a high-pressure capillary-tube viscometer operating in a high-shear-rate range such as that of a typical rocket injector so that shearthinning could be measured) indicated that the desired performance was attained both in adequate yield stress for storage immobility and in low apparent viscosity at high shear.

Propellant mixtures of the present invention have also been investigated with regard to in-service use; thus, behavior on ignition, rupture of tanks, and cleanup of spills has been qualitatively examined.

In connection with ignition, by remote handling, milliters of liquid MHF-1 (a mixed hydrazine fuel) was poured from a test tube onto a quantity of unmodified bromine trifluoride, The same quantity of liquid MHF-l was poured onto a quantity of a bromine trifluoride mixture according to the present invention (comprising bromine trifluoride and 2.5 weight percent carbon black of the herein-designated type). Motion; picture sequences of the reactions revealed that the modified and unmodified oxidizers had approximately the same ignition characteristics within the resolving power of the camera at a speed of 16 frames per second; thus, there was no apparent delay in ignition occasioned by modifying the oxidizer to form a propellant mixture according to the present in vention. Detonation did not occur when liquid MHF-l was poured on modified BTF.

Propellant mixtures of the present invention demonstrated reversible pseudoplasticity, i.e., under conditions Of relatively short relaxation time, the yield stress value possessed by these mixtures permitted them to regain their gel or gel-like form when under little or no external force; thus, they reassumed their gelor gel-like structure after having been rapidly squeezed from a container through an 0.0l-inch ID orifice, the jet of flowing material reforming and producing a pile.

When unmodified RFNA is diluted with water, a sudden release of a profusion of toxic nitrogen oxide fumes occurs. However, a pile of RFNA mixture of the present invention, when surrounded with water, reacted only at the gel perimeter while retaining the pile configuration; when stirred into large quantities of water, the mixture of the present invention was destroyed with a gradual release of nitrogen tetroxide, readily-controllable dilution and in-service safety features being demonstrated thereby.

Concentrated propellant mixtures of the present invention may be provided, to save shipping weight and space, for example, by incorporating in the mixtures an excess of carbon black (generally not over 15 weight percent, however, or less than 10 Weight percent). Dilution of the concentrate may be achieved by addition of sufficient liquid oxidizer to impart the desired consistency.

EXAMPLE I To red fuming nitric acid was added five weight percent of carbon black, said carbon black having been prepared from acetylene gas by continuous thermal decomposition and comprising colloidal-sized particles of carbon joined together in a chainlike fashion, addition of the carbon black being made gradually and the ingredients being vigorously stirred during the addition process until, at the end of approximately two minutes of adding and stirring, a gel or gel-like mass was formed. It was determined that the mixture, which was black in color, had a high viscosity at slow flow, while at high flow it had a low viscosity and was pumpable.

EXAMPLE II The procedure of Example I was duplicated, except that in place of the red fuming nitric acid there was substitnted inhibited red fuming nitric acid, while the same kind of carbon black obtained from the same source was present to the extent of 4.5 weight percent. The resultant mixture was in the form of a gel or gel-type mass which was black in color; it had a yield stress of 'at least 250 dynes per square centimeter, while at high flow (shear rate of 100,000 cm.- it had a low apparent viscosity (approximately 0.15 poise), and it was pumpable. In working with substantial quantities of this mixture, it was observed that there was considerably less fume removal problem than that normally experienced with inhibited RFNA; for example, when some of the material was wiped upon a piece of paper which would normally ignite on contact with the liquid oxidizer, ignition did not occur, probably because the IRFNA mixture of the present invention did not flow into and penetrate the large surface of paper fibers exposed. Likewise, it was observed that, whereas with unmodified IRFNA the space in its container above the liquid is filled with copious quantities of red fumes, the fumes were not observed with IRFNA of the present invention.

EXAMPLE III The procedure of Example I was duplicated, except that in place of the red fuming nitric acid mentioned there was substituted bromine trifluoride, while the same kind of carbon black obtained from the same source was present to the extent of six weight percent. The resultant mixture had a yield stress in excess of .200 dynes per square centimeter, demonstrating a high viscosity at slow flow and a low viscosity at high flow, and being pumpable. After mixing, the mixture had the same appearance as the mixture of Example I, being black in color and having a gel or gel-type mass conformation.

EXAMPLE IV To a half-and-half solution of bromine pentafluoride and chlorine trifluoride was gradually added four weight percent of the same kind of carbon black as was used in Examples I, II, and III, the ingredients being vigorously stirred during the addition process until, at the end of approximately two minutes of adding and stirring, a gel or gel-like mass was formed. It was determined that the mixture, which was black in color, had a high viscosity at slow flow, while at high flow it had a low viscosity and was pumpable.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to' the appended claims rather than to the foregoing specification as indicating the scope of the invention.

There is claimed:

1. A mixture having the property of being unpourable under storage or low shear conditions and of becoming fluid and flowing under increased shear consisting essentially of an intimate mixture of an inorganic liquid oxidizer and from two to ten weight percent of carbon black, said carbon black being prepared from acetylene gas by continuous thermal decomposition and comprising colloidal sized particles of carbon joined together in a chainlike fashion so that the mass of the carbon black is a persistent three-dimensional structure.

2. A mixture in accordance with claim 1 in which the liquid oxidizer propellant consists essentially of red fum ing nitric acid.

3. A mixture in accordance with claim 1 in which the liquid oxidizer propellant consists essentially of inhibited red fuming nitric acid.

4. A mixture in accordance with claim 1 in which the liquid oxidizer propellant consists essentially of bromine trifluoride.

5. A mixture in accordance with claim 1 which contains dispersed solid particulates selected from the group consisting of high energy and high density materials carried by a liquid oxidizer propellant which is interdispersed with the carbon black.

6. A mixture in accordance with claim 5 which includes a minor weight percentage of a surface active agent, said surface active agent being oxidation-resistant.

7. A process for preparing inorganic liquid oxidizer mixtures having the property of being unpourable under storage or low shear conditions and of becoming fluid and flowing under increased shear which comprises intimately mixing an inorganic liquid oxidizer with from two to ten weight percent of carbon black, said carbon black being prepared from acetylene gas by continuous thermal decomposition and comprising colloidal sized particles of carbon joined together in a chainlike fashion so that the mass of the carbon black is a persistent three-dimensional structure.

8. A process in accordance with claim 7 in which the intimate mixing is accomplished ultrasonically to the extent that the liquid oxidizer and the carbon black are substantially completely interdispersed.

References Cited UNITED STATES PATENTS Van Loenen.

Southern et a1. 44-7 Kanarek 23-157 Kolfenbach 14974 Haynes et a1 52-1 BENJAMIN R. PADGETT, Primary Examiner.

US. or. X.R. 

1. A MIXTURE HAVING THE PROPERTY OF BEING UNPOURABLE UNDER STORAGE OR LOW SHEAR CONDITIONS AND OF BECOMING FLUID AND FLOWING UNDER INCREASED SHEAR CONSISTING ESSENTIALLY OF AN INTIMATE MIXTURE OF AN INORGANCI LIQUID OXIDIZER AND FROM TWO TO TEN WEIGHT PERCENT OF CARBON BLACK, SAID CARBON BLACK BEING PREPARED FROM ACETYLENE GAS BY CONTINUOUS THERMAL DECOMPOSITION AND COMPRISING COLLOIDAL SIZED PARTICLES OF CARBON JOINED TOGETHER IN A CHAINLIKE FASHION SO THAT THE MASS OF THE CARBON BLACK IS A PERSISTENT THREE-DIMENSIONAL STRUCTURE.
 2. A MIXTURE IN ACCORDANCE WITH CLAIM 1 IN WHICH THE LIQUID OXIDIZER PROPELLANT CONSIST ESSENTIALLY OF RED FUMING NITRIC ACID. 